./x.py clippy: allow the most noisy lints

[matthiaskrgr]

This silences the following clippy lints in ./x.py clippy:

many_single_char_names (there are a lot of warnings caused by stdarch)
collapsible_if (can reduce readability)
type_complexity
missing_safety_doc (there are almost 3K warnings issued)
too_many_arguments
needless_lifetimes (people want 'tcx lifetimes etc)
wrong_self_convention (warns about from_..(), to_..(), into_..().. fns that do or do not take self by reference.

Just for clarification; this only changes the output of x.py clippy inside the rustc repo and does not change anything about clippy or how cargo clippy is run on peoples crates.

[rust-highfive]

r? @Mark-Simulacrum

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[jyn514]

collapsible_if (can reduce readability)

This one surprises me, I can't imagine when I'd want to have if { ... } else { if { ... } }

[matthiaskrgr]

collapsible_if also triggers on

if x {
   if y { }
}

and wants you to
if x && y { ...

[matthiaskrgr]

I tried to split up the lint into two so that we can keep linting the if x {} else { if y {} } case in the future: rust-lang/rust-clippy#6544

[Mark-Simulacrum]

FWIW, I can see wanting the nested if form - especially with a comment on the nested if in the else block. I also semi-frequently see the pattern of branching on one condition (e.g., something like if stage == 0 in bootstrap, and then the nested if being a special case. I want the structure to communicate that there is special behavior for stage 0, and a general case for all other stages. Under some other conditions the general case has a special case (e.g., maybe a config flag) but it feels better to have it nested - future refactoring would want to otherwise refactor the if into the else.

[Mark-Simulacrum]

Seems fine to me, I guess, but I don't feel strongly about clippy running in this repo.

@bors r+ rollup

[bors]

📌 Commit 6a4b24e has been approved by Mark-Simulacrum

[matthiaskrgr]

I've reduced nesting of if else if conditions in #80793